1. Field of the Invention
The present invention relates to a sliding member having a hard chrome-plating film formed on a sliding surface thereof.
2. Description of the Related Art
Sliding members such as a piston ring for an internal combustion engine, a cylinder liner sliding therewith, a rocker arm and a cam shaft are required to be free from seizure with a counterpart member, wear-resistant in the sliding member itself, and not to be aggressive against the counterpart member. To cope with these requirements, it is the conventional practice to apply a hard chrome plating excellent in wear resistance to the surface of a sliding member, particularly the outer peripheral surface of a piston ring.
However, simple application of chrome plating alone to the substrate of the piston ring cannot give a sufficient seizure resistance because the hard chrome-plating film is poor in oil retentivity. A common counter-measure against this problem is to form a number of mesh-shaped microcracks on the surface of the chrome-plating film by applying chrome plating, and then in the same chrome plating bath, conducting etching through application of a polarity reversing treatment. These microcracks serve as lubricant sumps, thus improving oil retentivity of the hard chrome plating film and imparting an excellent seizure resistance to the chrome-plated sliding member.
More recently, however, requirements for a lower fuel consumption and a higher output have become more strict, and along with this, the load imposed on various parts of internal combustion engines including piston rings is only increasing. As a result, some internal combustion engines cannot now sufficiently satisfy requirements for wear resistance, seizure resistance and fatigue strength even with a hard chrome-plating film having microcracks. More specifically, microcracks with a V-shaped cross-section are formed in the hard chrome-plating film of the piston ring through polarity reversing treatment of the film. Consequently, the hard chrome-plating film has initially a sufficient oil retentivity. However, as the thickness decreases as a result of progress of wear, the opening area and the total void volume of the microcracks decrease, this leading to deterioration of oil retentivity and resulting in shortage of seizure resistance. When the chrome-plated piston ring slides in the cylinder, stress concentrates with bottoms of V-shaped cracks (notches) of the hard chrome-plating film as bases, thus causing a so-called xe2x80x9cnotch effectxe2x80x9d and accelerating progress of cracks. This leads to deterioration of the film, and may finally cause breakage.
As a solution of such a problem, there is proposed a method comprising the steps of causing a hard chrome plating layer to precipitate into a small thickness through a positive polarity process onto the sliding surface of a sliding member, then forming a hard chrome plating thin-film layer having microcracks by causing a slight growth of the microcracks through a subsequent polarity reversing step, and sealing openings of the microcracks in a lower layer with the upper layer serving as the hard chrome plating thin-film layer by repeating these positive polarity step and the polarity reversing step, thereby forming a hard chrome plating film having many pores of microcracks independent in the film forming direction (Japanese Unexamined Patent Publication No. 10-53881).
According to the hard chrome plating film of this proposal, the notch effect caused by sliding operation does not reach the microcracks of the lowermost layer before the lowermost hard chrome plating layer is exposed as a result of progress of wear of the hard chrome plating film, thus improving fatigue strength. While the microcracks have a V-shaped cross-section in the both cases, these microcracks are formed in all the layers independently in the film forming direction. Therefore, even when the opening area of the microcracks and the total void volume once decrease along with the progress of wear, new microcracks appear every time the lower hard chrome plating layer is exposed, and oil retentivity recovers.
However, in a hard chrome plating film having microcracks independent in the film forming direction such as that disclosed in Japanese Unexamined Patent Application Publication No. 10-53881, substantially uniform size microcracks vacant pore groups are formed in all the layers. In other words, the bottom of each microcrack vacant pore group stops within the hard chrome plating layer where itself opens. In the film, therefore, the microcrack vacant pore groups are distributed in parallel along the direction in which hard chrome plating layers expand. Areas where a microcrack vacant pore group is non-existent in the film growing direction appear periodically. In this case, wear resistance and seizure resistance decrease periodically along with progress of wear of the hard chrome plating film.
As a solution of this problem, the aforementioned Japanese Unexamined Patent Application Publication No. 10-53881 discloses a method comprising the steps of forming a hard chrome plating layer having wavy swell like a bend in a stratum by applying a plating step after forming appropriate surface irregularities through honing or the like on the sliding surface of a sliding member, forming microcracks through a polarity reversing step, and subsequently, repeating the plating and polarity reversing steps. In a multi-layer hard chrome plating film formed by this method, microcrack vacant pore groups are distributed in wavy swell in match with bending of the layers. Even in progress of wear of the hard chrome plating film, therefore, microcracks always appear uniformly on the surface of the film.
However, in order to bend the hard chrome plating layers by this method, it is necessary to control the extent of surface irregularities of the interface with the substrate to be plated through a surface treatment such as honing. Even when appropriate surface irregularities are formed by applying a surface treatment such as honing to the interface with the substrate of the sliding member, bending becomes slower along with growth and multiplication of the hard chrome plating layers, and the hard chrome plating layer near the surface is relatively flattened, thus making it impossible to completely prevent periodical decrease in wear resistance and seizure resistance.
The present invention was developed in view of the circumstances as described above and has a first object to provide a chrome-plated sliding member which permits prevention of a periodical decrease in wear resistance and seizure resistance even without intentionally forming swell of the multi-layer hard chrome plating film while effectively making use of chrome plating conventionally available at a low manufacturing cost.
A second object of the invention is to provide a manufacturing method of a chrome-plated sliding member, which permits prevention of a periodical decrease in wear resistance and seizure resistance of a multi-layer chrome-plated film without the need to intentionally form surface irregularities on the interface with the substrate.
To achieve the aforementioned objects of the invention, the invention provides a chrome-plated sliding member having a hard chrome plating film comprising at least two hard chrome plating layers provided on the interface with a substrate, wherein microcracks opening to the outer surface side of the hard chrome plating layers distributed in the individual hard chrome plating layers; each of the microcracks in each hard chrome plating layer comprises a portion where the crack stops within a layer containing the opening thereof, and a portion where the crack propagates into the hard chrome plating layer under the layer containing the opening thereof, relative to the depth direction; and quantities of microcracks as expressed by area ratios of microcracks in a cross-section of the hard chrome plating film comprise a quantity of the portion where the crack stops within the layer containing the opening thereof within a range of from 1.5 to 35.0%, a quantity of the portion where the crack propagates into the hard chrome plating layer under the layer containing the opening thereof within a range of from 0.5 to 25.0%, and a total quantity of microcracks within a range of from 2.0 to 40.0%.
The manufacturing method of a chrome-plated sliding member provided by the invention comprises a hard chrome plating step of the interface with a substrate by use of a chrome plating bath, then an etching step by reversing polarity over a period of time within a range of from 100 to 2,000 micro-seconds, and repeating the hard chrome plating step and the etching process at least once, thereby forming a hard chrome plating film comprising at least two hard chrome plating layers.
In the chrome-plated sliding member of the invention, microcracks of the individual hard chrome plating layers forming the multi-layer hard chrome plating film have relatively shallow portions where bottoms thereof stop in the layer containing the openings thereof and relatively deep portions where cracks run through to reach a deeper position than the layer containing the opening thereof. Pores of microcracks are therefore present even in an area immediately before transfer from an upper layer to a lower layer of the individual hard chrome plating layers. Periodical decreases in oil retentivity do not therefore occur, or occurrence thereof, if any, causes only a slight decrease in oil retentivity. Stable wear resistance and seizure resistance are always ensured in use for a long period of time.
According to the manufacturing method of a chrome-plated sliding member of the invention, it is possible to manufacture a chrome-plated sliding member having excellent sliding performance as described above. According to the method of the invention, it is not necessary to intentionally form surface irregularities for bending the chrome-plated film on the interface with the substrate, thus permitting simplification of the surface treatment steps of the substrate.
According to the invention, particularly, there is available a chrome-plated piston ring excellent in wear resistance, seizure resistance and fatigue strength.